The Amiga BTX Kabel was an adaptor to connect Amigas to the German online information provider "BTX". The cable contains no circuitry but is really just an adaptor from the Amiga's parallel port to the BTX connector adaptor, the real work is done by the Amiga in software. BTX is an acronym for "Bildschirmtext" with means "on screen text" and is a similar technology to "Teletext". UK readers could probably equate BTX with the old service called "Prestel" that used to be supplied by "British Telecom". Later it was renamed to "Datex-J" and was a service provided by the German Post since 1984. The service was organised into information pages, most of which were static and contained an internal mailing system for BTX users. Technically a page consists of 40rows and 24lines of pure text with some graphics which was transmitted at 1200baud over a phone line.
BTW was intended to attract many "ordinary people" because it appeared more like a TV service rather than a computer service. Normally the user did not need a computer but used a BTX terminal.
Thanks to Sebastian Fahrner.